Photoconductive target

ABSTRACT

A photoconductive target comprising a transparent substrate, a transparent conductive layer, an N-type semiconductor layer, and a P-type photoconductive layer, each of the layers being successively provided on the substrate, the P-type photoconductive layer forming together with the N-type semiconductor layer a heterojunction and consisting of tellurium not exceeding 30 atomic %, arsenic ranging from 2 to 30 atomic % and the remainder of selenium, and the hetero-junction being reverse biased.

United States Patent Goto et al. Nov. 25, 1975 l l PHOTOCONDUCTIVE TARGET [75] inventors: Naohiro Goto; Yukinao lzozaki, [56] References Cited both of Machida; Keiichi Shidara, UNITED STATES PATENTS fl tf Eiichi Marutamafl'raqaaki 3346.755 10mm Dresner i. 313/65 A x HIraI, both of Kodatra; Klyohl a 3,350.595 10/1967 Kramer o v 313/65 A X lnan; lkue Mori, both of Hachioji, 3,405,298 lO/l968 Dresner 313/385 all of Japan [73] Assignees: Hitachi, Ltd.; Nippon Hose Kyokai, Primary Examinerwpfoben Segal both of Japan Attorney. Agent, or Firm-Craig & Antonelll [22] Filed: Nov. 22, 1972 [57] ABSTRACT i 1 PP bio-1308577 A photoconductive target comprising a transparent Rehned Applicafion Data substrate, a transparent conductive layer, an N-type [63] cominualiomnpan of Ser NO 35 8 A m 19 semiconductor layer, and a P-type photoconductive I97 abandoned p layer. each of the layers being successively provided on the substrate the P-type photoconductive layer [30] Foreign Application Priority Dam forming together with the N-type semiconductor layer A 22 [970 J a heterojunction and consisting of tellurium not exapan 4543767 ceeding 3O atomic arsenic ranging from 2 to 30 atomic and the remainder of selenium. and the [52] U.S. Cl. 313/366; 313/386; 313/94 heterofl-uncuon being reverse biased. [51] Int. Cl. HOlJ 29/45; H011 3l/38;

HOlJ 39/14 9 Claims, 7 Drawing Figures [58] Field of Search a 313/65 A U.S. Patent Nov. 25, 1975 Sheet 3 of3 3,922,579

A v 1 EmmmEEm 0 0 0 2'0 30 ATOM/C T9 17v Se-Te SYSTEM M m o a 4 w. A 3: RENEWED kwiq /0 20 ATOM/c As [/v 5e As SYSTEM & mo 4 2 0 Q5 kamfiwmmk 5% m3: 60% E mam mkmbu $3 L6 8 55% INVENTORS NAQHIRO GOTOIYUKINAO 1502A Kl, KEIICHI SH|1 ARA,EI|CH| HARUYAm A AAK! HIRI, KIYDHI- A INAOand IKUE M R ATTORNEYS PHOTOCONDUCTIVE TARGET CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part application of our copending application, Ser. No. 135,218 filed on Apr. [9, 1971, now abandoned.

FIELD OF THE INVENTION This invention relates to a photoconductive target of an image pickup tube.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a longitudinal cross-sectional view showing the construction of a conventional photoconductive image pickup tube.

FIG. 2a is a longitudinal cross-sectional view showing the construction of an example of the photoconductive target of this invention.

FIG. 2b is a longitudinal cross-sectional view showing the construction of another example of the photoconductive target of this invention.

FIG. 3 is a current vs voltage characteristic curve of the photoconductive target of this invention.

FIG. 4 is a diagram showing the composition ratio of the P-type photoconductive material used for the photoconductive target of this invention.

FIG. 5 is a characteristic curve showing the dependence of the photo-current (sensitivity) and dark current of a photoconductive target element, which comprises a hetero-junction formed by a P-type photoconductive material of selenium-tellurium binary mixture and an N-type cadmium selenide, on the doping amount of tellurium.

FIG. 6 shows the dark current and the residual photocurrent of a photoconductive target element, which comprises a hetero-junction formed by a P-type photoconductive material of a selenium-arsenic binary mixture and an N-type cadmium selenide, and the dependence thereof on the doping amount of arsenic.

DESCRIPTION OF THE PRIOR ART A construction of a photoconductive image pickup tube is indicated in FIG. 1, wherein I is a evacuated glass tube, 2 an electron gun generating a deflectable electron beam and 3 a photoconductive target which converts the image signal to the electric signal by scanning of the electron beam emitted from the electron gun.

The photoconductive target 3 which is provided on a transparent substrate 4, conventionally comprises a photoconductive material thin film provided on a trans parent conductive layer 5 so as to face the electron gun 2. Amorphous selenium has been used for the photoconductive material, but it is thermally unstable and gradually changes to gray crystalline selenium even at room temperature. The crystalline selenium has a far lower electric resistance than that of the amorphous selenium and, therefore, is not desirable as a vidicon target material. Therefore, as in U.S. Pat. No. 3,310,700, it has been attempted to prevent the crystallization of selenium by providing an antimony trisulfide (Sb S layer in at least one surface of the amorphous selenium or its alloy layers. Also, as in U.S. Pat. No. 2,687,484, there has been known an arrangement wherein in order to improve the low photosensitivity for red light of amorphous selenium, the photoconductive layer comprises a first layer of antimony trisulfide, cadmium sele- 2 nide or hexagonal crystalline selenium provided on a transparent conductive layer and a second layer provided on the first layer.

However, the above target does not always provide a desired junction characteristic because, due to the dependence of the response speed on the life time of free carriers, there exists a reciprocal relationship between the response speed and photosensitivity one of which must be sacrificed.

SUMMARY OF THE INVENTION This invention is intended to eliminate the above defects in the photoconductive targets of the TV pickup tube and is directed to providing a photoconductive target having a low dark current, a high sensitivity for visible light and also a higher long wavelength limit than an amorphous selenium, etc.

DESCRIPTION OF THE PREFERRED EMBODIMENT The photoconductive target ofthis invention, as indicated in FIG. 2a or FIG. 2b, is a photoconductive target element comprising a first P-type photoconductive material layer 3 consisting of tellurium not exceeding 30 atomic arsenic ranging from 2 to 30 atomic Z and the remainder of selenium and a second N-type semiconductor layer 3., said first layer and second layer forming a hetero-junction and being reverse biased. The second N-type semiconductor material preferably employed in this invention includes zinc selenide, cadmium sulfide, cadmium selenide, zinc sulfide. gallium arsenide, indium oxide, tin oxide, titanium oxide. germanium, silicon, etc., cadmium selenide. gallium arsenide, germanium and silicon in particular being capable of improving the sensitivity for red light. And. as indicated in FIG. 2a, the provision ofa third layer 3 of antimony trisulfide, arsenic triselenide or the compound thereof may be provided on the first P-type semiconductor material layer to inhibit secondary electron emission from the selenium, tellurium and arsenic, thereby preventing the occurrence of undesirable phenomena such as ripple-like beam landing, high speed beam landing, etc.

This invention will now be described with reference to the examples.

To produce the first P-type photoconductive material described above, a raw material consisting of atomic 7r selenium (Se), 10 atomic Zr tellurium (Te) and I0 atomic 7r arsenic (As) is ground and mixed in an inert atmosphere, and the mixture thus produced is enclosed in a quartz ampoule which is then evacuated to a pressure of about 10" Torr and then heated in an electric furnace at IOOOC for over 3 hours to be fused and then rapidly cooled in air to obtain a glass-like photoelectric material. On the other hand, a glass substrate provided with a transparent electrode is heated to 200C and under a pressure of about 2 X I0 Torr, zinc selenide (ZnSe) is vapor deposited to a thickness of about 2000 A on the glass substrate to form a film-like N-type material. Then. on the film-like N-type material thus produced the glass-like photoelectric material obtained above is deposited by vacuum evaporation to a thickness of 2 to 5p. under a pressure of about 2 X 10 Torr at room temperature to form a hetero-junction therewith. The photoconductive layer thus obtained is incorporated, for instance, in a vidicon image pickup tube as a target and the transparent electrode described above is positively biased relative to the cathode so that the hetero-junction is reverse biased,

When the photoconductivc target element of this invention is constructed as described above, the heterojunction formed has a current vs voltage characteristic as indicated by a curve of PK]. 3. In general, the reverse current of a junction is smaller than that of the ohmic current of the parent material and the photo re sponse of the reverse current. being dependent on the transit time of the carrier across the junction. is faster than that of the ohmic photocurrent of the parent material. Therefore, the photoconductivc target, with the junction thereof reversely biased, has a low dark current and a fast response speed and comprises a heterojunction of parent materials having different sensitive wavelength bands and hence by virtue of prevention of the decrease of photocurrent due to surface recombination, has a wider sensitive wavelength band than ei ther the P-type material or the N-type material. And, although vacuum evaporated films ofpolycrystallinc or amorphous semiconductor material do not generally exhibit a good junction characteristic, many of the selenium base materials exhibit a relatively good junction characteristic That is, amorphous selenium usually indicative of P-type, in contact with an N-type material, exhibits a rectification characteristic similar to that of a P-N junction and. in contact with a P material, exhibits a characteristic similar to that of the P-P junction The photoelectric material employed in this invention contains. in addition to the selenium, tellurium and arsenic, whereby the inclusion of tellurium contributes to increase the photosensitivity for the long wavelength light and the inclusion of arsenic contributes to improve the characteristic of the hetero-junction so that the photosensitivity for the long wavelength light is increased, and the crystallization of the glassy state of the photoelectric material is also prevented thereby extending its service life. However, the photoelectric material, when containing large amounts of arsenic, tends to exhibit the accumulation effect of photoconductivity and thus even removal of the irradiation, does not restore its original dark resistance, and when containing large amounts of tellurium, has its specific resistance lowered thereby leading to a large reverse current in the junction formed and tends to have reduced S/N ratio as a photo detector. In view of these points, the contents of both elements must be kept below 30 atomic respectively, of the whole. The composition range meeting this requirement is indicated in a Gibbs triangle of FIG. 4.

in FIG. 4, the apexes of the Gibbs triangle represent selenium (Se). tellurium (Te) and arsenic (As). the numerals showing atomic 7'. Furthermore, a selenium-tellurium binary photoconductive material (P-type) not containing arsenic and an N-type conductive material are provided on a transparent conductive film to form a hetero-junction target. The dependences of photocurrent (p. A) and dark current of this target on the doping amount of tellurium are indicated in FIG. 5, wherein the abscissa represents the doping amount of tellurium by atomic 7% in the selenium-tellurium system. and curve D indicates the dark current and curve P photocurrent. As seen from FIG 5, the photocurrent, i.e. sensitivity increases as the content of tellurium increases, but with an accompanying increase of the dark current and, therefore, in order to retain the dark current below 10 nA, the upper limit of the doping amount of tellurium is 30 atomic 7?. Furthermore, a seleniumarsenic binary photoconductive material (P-type) complctely devoid of tellurium and an N-typc cadmium selenide are provided on a transparent conductive film to form a hetero-junction target. The change of the dark current due to heat treating this target at for l hour and the residual photocurrent after 10 seconds after interruption of the irradiation to the target are shown, in the dependence on the doping amount of arsenic. in FIG. 6, wherein D' indicates dark current and P photocurrcnt, and the abscissa represents the doping amount of arsenic by atomic 7: in the selenium-arsenic binary system. As seen from FIG. 6. by the doping of arsenic, the increase of the dark current due to heat treatment can be inhibited, but an excessive amount of arsenic increases the residual photocurrent after the interruption of light and, therefore, the upper limit of amount of arsenic is 30 atomic And. the lower feasi ble limit of the amount of tellurium for a TV pickup tube material can be reduced to 0%. When the doping amount of arsenic in the P-type photoconductive layer is extremely small, white points generate in the image to be obtained. There is shown in the following Table the relations between the amount of arsenic doped into the P-type photoconductive layer and the increase of dark current and the number of white point generated by heating the target for an hour at 60C.

Doping amount of arsenic 0 l 2 3 4 5 6 (atomic '7r] Increasing extent of dark current l0 5 3 2 l,5 l 0.8

Number of white points generated I6 I O 0 l 0 The target according to the present invention is operated within normally less than 5 nA or at most l0 nA of dark current, but as can be seen from the above Table, when the increase of the dark current gets nearer to the upper limit of the initial dark current, a great number of white points generate on the image, and as a result the target can not substantially function as a target for an image pick-up tube. Therefore, the lower limit of the doping amount of arsenic applicable for practical use is 2 atomic 7:.

Although in the above example only cadmium senenide is illustrated as the N-type semiconductor. other N-type semiconductors such as cadmium sulfide, zinc sulfide, zinc selenide, gallium arsenide, indium oxide, tin oxide, titanium oxide, germanium and silicon can also be used with the same effectiveness. The dark current, photocurrent for visible light, life and lag of the TV pickup tube employing the photoconductive target of this invention, as compared to those of the TV pickup tube employing a conventional antimony trisulfide photoconductive target, are indicated in the following table.

As is evident from the above table. the TV pickup tube according to this invention has a smaller dark current and, particularly, a higher photosensitivity as compared to the conventional photoconductive TV pick-up tube,

The photoconductive target element of this invention has a very simple construction, an improved spectral sensitivity. a wider sensitive wavelength band and a low dark current as well as a large S/N ratio, using it a TV pickup tube having a stable and long operational life can be fabricated.

We claim:

1. A photoconductive target comprising a transparent substrate having a flat surface, a transparent conductive layer formed on said substrate, an N-type semiconductor formed on said conductive layer, and a P- type photoconductive layer consisting of tellurium not exceeding 30 atomic arsenic ranging from 2 to 30 atomic and the remainder of selenium provided on said N-type semiconductor layer and forming a heterojunction therewith, said heterojunction being reverse biased, said N-type semiconductor layer comprising a material selected from the group consisting of zinc selenide, cadmium sulfide, cadmium selenide, zinc sulfide, gallium arsenide, indium oxide, tin oxide, titanium oxide, germanium and silicon.

2. A photoconductive target according to claim 1, further comprising a thin layer of a material selected from the group consisting of antimony trisulficle and arsenic triselenide extended on said P-type photoconductive layer.

3. A photoconductive target according to claim I wherein said P-type photoconductive layer consists of atomic 7e selenium. l0 atomic 7r tellurium and 10 atomic 7r arsenic.

4. A photoconductive target according to claim 1, wherein said P-type photoconductive layer includes arsenic in the range from l2 to 30 atomic "/1.

5. A photoconductive target according to claim 1, wherein said P-type photoconductive layer includes ar senic in the amount of 10 atomic 7c.

6. A photoconductive target according to claim 1, wherein said P-type photoconductive layer includes arsenic in the amount of 3 atomic 7. A photoconductive target according to claim I, wherein said P-type photoconductive layer includes arsenic in the amount of 4 atomic 7c,

8. A photoconductive target according to claim I, wherein said P-type photoconductive layer includes arsenic in the amount of 5 atomic 7r.

9. A photoconductive target according to claim I, wherein said P-type photoconductive layer includes arsenic in the amount of 6 atomic 7r.

* l I K 

1. A PHOTOCONDUCTIVE TARGET COMPRISING A TRANSPARENT SUBSTRATE HAVING A FLAT SURFACE, A TRANSPARENT CONDUCTIVE LAYER FORMED ON SAID SUBSTRATE, AND N-TYPE SEMICONDUCTOR FORMED ON SSAID CONDUCTIVE LAYER, AND A P-TYPE PHOTOCONDUCTIVE LAYER CONSISTING OF TELLURIM NOT EXCEEDING 30 ATOMIC %, ARSENIC RANGING FROM 2 TO 30 ATOMIC % AND THE REMAINDER OF SELENIUM PROVIDED ON SAID N-TYPE SEMICONDUCTOR LAYER AND FORMING A HETERO-JUNCTION THEREWITH, SAID HETERO-JUNCTION BEING REVERSE BIASED, SAID N-TYPE SEMICONDUCTOR LAYER COMPRISING A MATERIAL SELECTED FROM THE GROUP CONSISTING OF ZINC SELENIDE, CADMIUM SULFIDE, CADMIUM SELENIDE, ZINC SULFIDE, GALLIUM ARSENIDE,
 2. A photoconductive target according to claim 1, further comprising a thin layer of a material selected from the group consisting of antimony trisulfide and arsenic triselenide extended on said P-type photoconductive layer.
 3. A photoconductive target according to claim 1 wherein said P-type photoconductive layer consists of 80 atomic % selenium, 10 atomic % tellurium and 10 atomic % arsenic.
 4. A photoconductive target according to claim 1, wherein said P-type photoconductive layer includes arsenic in the range from 12 to 30 atomic %.
 5. A photoconductive target according to claim 1, wherein said P-type photoconductive layer includes arsenic in the amount of 10 atomic %.
 6. A photoconductive target according to claim 1, wherein said P-type photoconductive layer includes arsenic in the amount of 3 atomic %.
 7. A photoconductive target according to claim 1, wherein said P-type photoconductive layer includes arsenic in the amount of 4 atomic %.
 8. A photoconductive target according to claim 1, wherein said P-type photoconductive layer includes arsenic in the amount of 5 atomic %.
 9. A photoconductive target according to claim 1, wherein said P-type photoconductive layer includes arsenic in the amount of 6 atomic %. 